When a sink doesn’t drain properly even when it’s not clogged, it can be difficult to triage the problem. Often the issue actually relates to plumbing fixtures the typical homeowner may not be aware of. 

A slow-draining sink without a visible clog is most likely caused by a blockage beyond the P-Trap or an incorrectly installed P-Trap. Other potential issues include the lack of a vent, a blocked vent, an outdated plumbing system, or tree roots infiltrating your plumbing wastage system. 

I grew up in an old Victorian house with clay pipework and as a result, we experienced sink and drainage blockages weekly. I have firsthand experience with the most common causes of sinks draining poorly, even without a visible blockage. I’m going to break down for you how to easily fix the issue.

Why Won’t Your Sink Drain Even Though It’s Not Clogged?

When a sink is seemingly not clogged yet won’t drain, there are a few common causes:

• A blockage further down the system
• Incorrect P-Trap installation
• A blocked Air Admittance valve, or a blocked vent
• Old and outdated pipework
• Tree root infiltration

I’ll walk you through each of these reasons in detail. I’ll talk you through the signs to look out for as well as how you can solve your slow-draining sink. 

Pro tip: Sinks drain much slower when the faucet is flowing directly into the sink strainer. Try swinging the faucet a bit which allows the drainage to breathe.

Sink Unclogging: Have You Gone Deep Enough?

Kitchen sinks in particular are prone to blockages further down the system than many homeowners realise. This is because they frequently encounter a combination of food particles, grease, and soap residue which can accumulate and solidify within the pipes over time. 

Blockages can occur several feet down the pipes (my record is finding a blockage 15 feet beyond the kitchen sink) so before moving on to the subsequent sections I’d recommend unclogging as far as possible and within this section, I’ll explain how. 

Firstly, check your P-Trap is fully clear of blockages. The P-Trap sits just underneath your sink and prevents sewer gas from entering your kitchen (or bathroom) while allowing waste water to flow freely. Food waste can easily get caught in the P-Trap so you should first try unblocking it using a plunger. Pour hot water down the sink and then plunge it using an up-and-down motion. 

Plunging is the most effective but you can also try the baking soda and vinegar combination. Pour a cup of baking soda and a cup of vinegar down the sink, wait 5 minutes, and then flush with hot water. Repeat as necessary. 

Failing this, you will need to remove the P-Trap to ensure it’s free of blockages. Pro tip: If you do remove the P-Trap I’d recommend installing a clear P-Trap (on Amazon). It enables you to detect blockages and effectively clean them.  

Here’s an instructional video showing you remove and clean the P-Trap: 

Practical Tips: Using a Plumbing Snake for Deep Sink Clogs 

Assuming your P-Trap is clear, turn your attention to further down the plumbing system. It’s possible your blockage is beyond the P-Trap and many feet away. For this, you will need a plumbing snake (on Amazon). 

A plumbing snake is a long flexible tool that is inserted into drain pipes and clears blockages by either breaking them up or extracting them. I’d recommend wearing gloves and goggles as this can be a slightly messy job! 

You’ll first need to access the wastage pipe within the wall rather than on the sink. To do this you’ll need to remove your plumbing beneath the sink. Turn off the water supply valves beneath the sink and disconnect the P-Trap and other connecting pipes. You’ll need to rotate the slip nuts anti-clockwise and normally this is doable by hand but if not then use a wrench. 

1. Insert the auger end of the snake into the drain and feed the cable in. The plumbing snake has a locking mechanism that you’ll need to continually lock and unlock whilst pushing the cable further and further down the drain while rotating it using the handle. 
2. If the snake encounters a blockage, crank the cable around a few times to catch it, then pull out both the cable and the debris. When you feel resistance, slowly withdraw the cable, pulling out the clog.  Note: Be prepared for a really bad smell! 
3. If you don’t feel a blockage, keep feeding the snake down whilst rotating using the handle. 
4. Once you’re done, you’ll need to withdraw the snake back into its casing. To do this, you’ll need to crank the handle and clean the snake whilst recoiling it back into the casing. 
5. Run water down the drain to flush out any remaining debris the snake may have dislodged. 

Incorrect P-Trap Installation: A Common Plumbing Headache

The P-Trap sits underneath your kitchen or bathroom sink and it prevents sewer gases from entering the home. 

If the vent is clogged, if there is no vent, or if the P-Trap has been installed incorrectly, your sink will not drain as it should, even when it’s not clogged.

P-Traps must be installed with a vent. Without a vent (or AAV) there will be suction inside the trap as the water drains. This could also cause a smell as water from the trap would be removed, and nasty gasses could enter the home. 

The best way to tell if you have a vent is to visually inspect the pipes and to see any venting pipes attached to the drainage beyond the P-Trap. If not, either installing a vent or an AAV could fix the issue. Note: Certain states have regulations on the usage of AAV and this is probably a job for a qualified plumber. 

Secondly, your P-Trap may be installed incorrectly. The most common issue that may affect drainage is where the drainage pipe beyond the P-Trap does not slope down steep enough to remove water at the required volume. 

This means that your sink may produce wastewater at a quicker rate than your waste plumbing is able to drain. The waste pipes underneath your sink should either move vertically down, or horizontally downwards at ¼ inch per foot of pipe. 

Note: Oversloping drain pipes are just as likely to clog as undersloping pipes. 

The Sink Air Admittance Valve and Vent Clogs  

Sinks must be installed with either air admittance valves (AAV) or vents. An AAV is a one-way valve used to vent plumbing fixtures while a vent is a dedicated pipe that extends from plumbing fixtures to the outdoors. If either of these components gets blocked your sink will struggle to drain without the sink itself being blocked. 

Blocked Air Admittance Valve 

AAVs can fail due to a number of reasons including old age, dust build-up, or faulty installation. If this is the case, a vacuum may be created within your sink’s drainage plumbing preventing the waste water from draining as effectively as it could. 

If this is the case you may also notice nasty smells, strange noises coming from the valve, and leaking water. If this is the case you’ll need to replace your AAV (on Amazon) but luckily they are cheap and can simply be screwed on. 

Blocked Plumbing Vent

A vent is a plumbing fixture (required by plumbing codes in many states) and allows air from your waste pipes to escape. This allows for a normal flow of wastewater. 

Vent stacks can be seen on the top of buildings and I have shown a picture below. Vents can get clogged due to a number of reasons but the most common are things like leaves or twice, animal nests, and ice or snow. 

In addition to your sink not draining, you may also notice the following things: 

• Water takes a long time to drain from the sink, in addition to other sinks or the shower within the home
• The sink makes gurgling sounds 
• The sink omits a foul smell
• Water backs up into other sinks. For example, maybe your bathroom sink fills up with water as your kitchen sink is trying to drain
• Bubbling toilets when flushed

With a blocked vent, other plumbing fixtures within the home may also be slow to drain. 

How to fix a blocked vent 

The most effective approach for clearing a plumbing vent is accessing your roof, but if your roof is hazardous or steep, get professional help. 

To check for a blocked vent pipe, cover the vent stack with your hand as an assistant flushes a toilet; if you don’t detect suction, it’s also a reliable sign of a blockage.

From there, you have two options: 

1. Use a plumbers snake (on Amazon). Apply some pressure to unblock anything you might find. 
2. Use a garden hose. If your hose stretches far enough to the roof, you can give the vent a blast of water for around 5 minutes. 

If you can’t access the vent from the roof another option you have is clearing the vent from the attic. All vents will lead to a large drain called the main stack which is the one you’ll find on the roof.  

1. Use a hacksaw to create a small opening in the vent, just large enough to accommodate threading a plumbing snake. 
2. Introduce the snake and direct it downward until you feel it encountering the blockage; rotate the handle continuously to remove the obstruction. 
3. Once the clog is removed, the hole in the PVC pipe will need repair; use PVC coupling (available on Amazon) to securely connect the cut ends of the vent pipe.

Old Pipes are causing slow drainage  

Old plumbing fixtures can lead to many issues and this is often the case in older homes. 

Sewer lines experience significant wear and tear. Pipes in older homes were constructed prior to the widespread use of modern appliances like garbage disposals, dishwashers, and pressurized power showers. As a result, they are more likely to cause drainage issues, particularly when extensive remodeling has occurred. 

In a typical household, the primary drainage pipe gathers wastewater from various fixtures and appliances, such as sinks, toilets, showers, and washing machines. It then carries this wastewater to either the sewer or the septic system. Most modern main drainage pipes have a standard diameter of 4 inches, although in some older homes, you might still find drain pipes as narrow as 1 inch. 

With an inadequate main drainage pipe, you’ll likely see sluggish water flow in the sink, particularly when other appliances like the dishwasher are in use. This can also lead to water pooling in the sink basin and may result in backups in other plumbing fixtures. Additionally, you may encounter more frequent occurrences of clogged drains.

Another issue that occasionally arises in older homes is the development of what’s known as “pipe bellies.” This occurs when pipes shift, creating unfavorable slopes. These bellies can trap water, encourage blockages, and even attract tree roots. Consequently, you might experience recurrent clogs and a slow-draining sink, even when it’s not obstructed.

If you suspect either of these scenarios, it’s advisable to seek the guidance of a plumber and consider a plumbing revamp. You may need to upgrade the undersized main drainage pipe or reconfigure the entire plumbing system.

Tree roots disrupting your drainage

Tree roots can create various issues for plumbing drainage systems. Roots are naturally attracted to the moisture and nutrients found in sewage pipes. They can sneak into small cracks or joints within the pipes and grow inside, leading to blockages. This can result in slower drainage and backups.

Even if the roots don’t entirely clog the pipe, their presence can narrow the passage for wastewater, reducing the system’s effectiveness and potentially causing overflows.

If you live near trees or have older sewer pipes constructed from clay or concrete, unfortunately, you may be at a higher risk of experiencing tree root damage.

Do you have other drains further down the drainpipe from your sink? If those drains are also draining slowly and you hear gurgling sounds from them while you’re using the sink, it’s possible that a sewer pipe extending from your home has suffered from tree root damage.

If this is the case you’ll need to get your sewer lines replaced and unfortunately it’s not cheap. It’s likely to cost you around $500 for the tree root removal and a further $50 – $200 per foot to replace damaged pipes.